The alert over Cadbury products contaminated with salmonella widened yesterday as it emerged other food companies bought chocolate crumb from the Herefordshire factory at the heart of the crisis.

After a meeting with the authorities in London yesterday, it also emerged Cadbury has only now agreed to a comprehensive cleaning of all the production lines at the Marlbrook plant concerned.

It first discovered it had a salmonella problem at the site in January this year, although the authorities believe that previous outbreaks in 2002 at its other factories may be traced back to Herefordshire. Cadbury only admitted to the contamination after an alert from the Health Protection Agency about an unusual rise in human cases of Salmonella montevideo. It agreed to recall more than 1m bars of seven types of chocolate brands that had tested positive two weeks ago.

The Guardian has learned that third- party companies also took supplies of the base ingredient from the Marlbrook plant for the manufacture of foods containing chocolate, although the Food Standards Agency was unable, last night, to provide a list of those involved.

Environmental health officers were checking Cadbury warehouses yesterday to see how much stock they held from the period when they say salmonella-contaminated crumb was fed into the processing of about 30 Cadbury brands other than the seven already recalled. Birmingham council's food safety team leader, Nick Lowe, confirmed he was concentrating on checking stock between January 22 and February 21 this year, when crumb from Herefordshire and products made at the Bourneville site in his area tested positive for the rare montevideo strain.

During that time he said approximately 150 tonnes of chocolate a day were made at the factory, suggesting that up to 4,500 tonnes could have been made at the Bourneville factory using contaminated crumb. So far, Cadbury is believed to have recalled about 250 tonnes. The FSA said it believed that the recall of the seven brands was "proportionate". The FSA said it had been reassured by Cadbury that crumb had only been dispatched to third parties once tests came back negative.

In contrast, crumb being sent to Cadbury's factories for further processing, was released on a "just-in-time" ordering system, so test results came back only after the ingredient had been sent on to other sites, and fed into the manufacturing process, according to Herefordshire's head of environmental health, Andrew Tector. Advice from the FSA's microbiological safety committee, however, has called into question Cadbury's tests. Testing final products could not guarantee absence of salmonella, it concluded.

Tim Lang, professor of food policy at City University in London said the alert was a test of whether the FSA, under its new management, was able to stand up to commercial interests.

Cadbury issued a statement after its meeting with the FSA and experts from Herefordshire council, saying that it would change its cleaning regimes at Marlbrook and increase testing. "We regret any concern the recent recall may have caused ... we have always acted in good faith," the managing director, Simon Baldry, said.